The hydroxyphenylpyruvate dioxygenases (HPPDs) are enzymes that catalyze the reaction in which para-hydroxyphenylpyruvate (HPP) is transformed into homogentisate. This reaction takes place in the presence of enzyme-bound iron (Fe2+) and oxygen. Herbicides that act by inhibiting HPPD are well known, and include isoxazoles, diketonitriles, triketones, and pyrazolinates (Hawkes “Hydroxyphenylpyruvate Dioxygenase (HPPD)—The Herbicide Target.” In Modern Crop Protection Compounds. Eds. Krämer and Schirmer. Weinheim, Germany: Wiley-VCH, 2007. Ch. 4.2, pp. 211-220). Inhibition of HPPD blocks the biosynthesis of plastoquinone (PQ) from tyrosine. PQ is an essential cofactor in the biosynthesis of carotenoid pigments which are essential for photoprotection of the photosynthetic centres. HPPD-inhibiting herbicides are phloem-mobile bleachers which cause the light-exposed new meristems and leaves to emerge white. In the absence of carotenoids, chlorophyll is photo-destroyed and becomes itself an agent of photo-destruction via the photo-generation of singlet oxygen.
Methods are also known for providing plants that are tolerant to HPPD herbicides and have included: 1) overexpressing the HPPD enzyme so as to produce quantities of HPPD enzyme in the plant that are sufficient in relation to a given herbicide so as to have enough of the functional enzyme available despite the presence of its inhibitor; and 2) mutating the target HPPD enzyme into a functional HPPD that is less sensitive to herbicides. With respect to mutant HPPDs, while a given mutant HPPD enzyme may provide a useful level of tolerance to some HPPD-inhibitor herbicides, the same mutant HPPD may be quite inadequate to provide commercial levels of tolerance to a different, more desirable HPPD-inhibitor herbicide (See, e.g., U.S. App. Pub. No. 2004/0058427; and PCT App. Pub. Nos. WO 98/20144 and WO 02/46387; see also U.S. App. Pub. No. 2005/0246800 relating to identification and labelling of soybean varieties as being relatively HPPD tolerant). For example, HPPD-inhibitor herbicides may differ in terms of the spectrum of weeds they control, their manufacturing cost, and their environmental benefits.
Accordingly, new methods and compositions for conferring HPPD herbicide tolerance upon various crops and crop varieties are needed.